The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.
During a borehole drilling operation there is a need to survey the path of the borehole to determine if the trajectory is being maintained within acceptable limits. Surveying a borehole is usually accomplished using a surveying tool which is moved along the borehole to obtain the information required, or at least data from which the required information can be determined. Information in relation the path of a borehole can typically include inclination, azimuth and depth.
Surveying tools typically contain sensor devices for measuring the direction and magnitude of the, local gravitational field, and also the rate of rotation of the Earth. These measurements correspond to the position and orientation of the surveying tool in the borehole. The position, inclination and/or azimuth can be calculated from these measurements.
The sensor devices-can comprise accelerometers for measuring the direction and magnitude of the local gravitational field, and gyroscopes for measuring the rate of rotation of the Earth, from which azimuth can be calculated.
Commercially available gyroscopes contain systematic errors which can seriously affect the accuracy of measurement.
With a view to eliminating, or at least reducing the systematic errors, it is known to index gyroscopes through 180 degrees between two indexing positions, with measurements being taken at the two indexing positions. Because the indexing positions are 180 degrees apart, the measurements will be reversed; that is, the measurements deliver the same data but with reversed polarity. With these measurements, the systematic errors can be eliminated or diminished.
Commercially available accelerometers also contain systematic errors which can be handled in a similar way.
In order to index sensor devices, such as gyroscopes and/or accelerometers, between various indexing positions, there is a need for an indexing mechanism aboard the surveying tool.
There is also a need to orient the sensor devices so that two orthogonal sensitive axes occupy a selected plane, which typically is horizontal.
The need to index and orient the sensor devices can introduce cost and complexity to the surveying tool, and can be particularly problematic where a survey tooling of compact construction is required.
It is against this background, and the problems and difficulties associated therewith, that the present invention has been developed.